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Geology
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Earth Oddities
Some fires burn where you cannot see them.
Beneath towns, forests, peatlands, mines and mountains, combustion can creep through buried fuel for
months, decades or even centuries.
These underground fires may reveal themselves only through hot ground, smoke escaping
from cracks, sulfurous odors, dying vegetation, glowing fissures or slowly collapsing terrain.
Some are coal seam fires. Others burn inside abandoned mines, peat deposits, buried waste
or gas-bearing ground. Some were started by people; others can ignite naturally.
And once established underground, they can become extraordinarily difficult to stop.
This guide explains how underground fires start, why they can burn for so long, how they move,
what hazards they create, how scientists detect them and why places such as Centralia, Jharia and Burning
Mountain became famous examples of fire beneath Earth’s surface.
Underground Fires in 60 Seconds
-
Underground fires burn beneath the surface in coal, mines, peat, buried waste or
gas-bearing ground. -
Coal seam fires can persist for decades or much longer when oxygen continues reaching
underground fuel. -
Peat fires can smolder below ground after surface flames disappear and may survive
rain, cold weather or even snow cover. -
Underground combustion can produce carbon monoxide, carbon dioxide, smoke, particulates and
other hazardous gases. -
Burning material can leave voids and weaken rock or soil, contributing to
subsidence, fissures and sudden collapse. -
Scientists monitor underground fires using ground temperature measurements, gas sampling,
boreholes, thermal cameras, drones and satellites. -
Extinguishing a large subsurface fire can be extremely difficult because the burning zone may be
inaccessible and connected to multiple underground air pathways. -
Famous examples include Centralia in Pennsylvania, the Jharia coalfield in India and Burning
Mountain in Australia.
The defining feature is simple: the visible smoke or hot ground is often only the surface
expression of a much larger combustion system underground.
What Are Underground Fires?
Fire requires three basic ingredients:
- fuel;
- oxygen;
- enough heat to sustain combustion.
Underground environments can contain enormous quantities of fuel.
Coal seams may extend for kilometers. Peat can accumulate several meters thick. Old mines expose coal
surfaces and create tunnels that transport air. Landfills contain combustible waste and generate gases.
If combustion reaches those materials and sufficient oxygen remains available, a fire can continue below
the surface long after any visible flames disappear.
Why underground fires look so strange
Unlike a normal wildfire, an underground fire may have no obvious flame front.
Instead, the landscape can show:
- smoke rising from cracks;
- steam or gas vents;
- warm or scorched ground;
- patches where vegetation suddenly dies;
- holes opening above burned material;
- glowing fissures at night;
- roads buckling or sinking;
- persistent smoky or sulfur-like odors.
That combination often makes underground fires look volcanic.
Most are not.
How Do Underground Fires Start?
There is no single ignition mechanism.
Underground fires can originate naturally or through human activity.
Spontaneous combustion
Some carbon-rich materials slowly react with oxygen even at relatively low temperatures.
Oxidation releases heat.
If that heat accumulates faster than it can escape, temperatures may rise until the material begins
smoldering or burning.
Coal is particularly important because oxidation and self-heating can contribute to spontaneous ignition
under favorable conditions.
Wildfire ignition
A surface wildfire can ignite exposed coal, peat or organic-rich soil.
Surface flames may later disappear while combustion continues underground.
Mining activity
Mining exposes previously buried fuel to oxygen and creates fractures, tunnels and waste piles.
Sparks, equipment, blasting, abandoned fires and spontaneous combustion can all provide ignition sources.
Lightning
Lightning can ignite vegetation or exposed combustible material, which may subsequently spread into
subsurface coal or peat.
Burning waste
Fires in dumps, landfills or waste piles can migrate below the surface into deeper combustible material.
Human negligence
Brush burning, discarded hot material, campfires and industrial activity can sometimes provide the initial
heat source.
Why Can Underground Fires Burn for Decades—or Longer?
The longevity of underground fires seems extraordinary until the geometry of the problem is considered.
Huge fuel reserves
A coal seam can contain millions of tons of combustible material.
Even a very slowly advancing fire may therefore have enough fuel to continue for generations.
Fractures supply oxygen
Cracks, mine tunnels, porous rock and collapsed ground create pathways allowing air to reach the fire.
Heating itself may fracture surrounding material, creating additional air routes.
Insulation
Soil and rock above the fire can act as insulation.
Rather than allowing heat to escape rapidly into the atmosphere, underground material retains heat and
helps sustain smoldering combustion.
Deep fires are difficult to reach
Firefighters cannot simply spray water onto a combustion front that may be tens of meters underground
and spread across an irregular network of seams and tunnels.
The fire can migrate
Even if one area is extinguished, heat may have already propagated into another section of the fuel layer.
Underground fires are therefore often three-dimensional moving systems, not stationary
holes filled with flame.
Types of Underground Fires
The phrase underground fire includes several very different processes.
| Type | Main Fuel | Typical Setting | Main Hazards |
|---|---|---|---|
| Coal seam fire | Coal | Coal-bearing rock | Smoke, gases, subsidence, collapse |
| Mine fire | Coal or mine materials | Active or abandoned mines | Toxic gases, heat, structural instability |
| Peat fire | Partially decomposed organic matter | Peatlands and organic soils | Persistent smoke, carbon emissions, ground loss |
| Landfill fire | Buried waste | Landfills and waste dumps | Toxic smoke, gas migration, instability |
| Natural gas fire | Hydrocarbon gases | Gas seeps, wells, crater systems | Fire, gas exposure, explosion risk |
Coal Seam Fires
Coal seam fires are among the most persistent underground fires on Earth.
They occur when a coal deposit ignites and combustion propagates through the seam.
Coal may burn:
- deep underground;
- along an exposed seam;
- inside abandoned mine workings;
- within mining waste piles;
- along fractured slopes where oxygen reaches buried fuel.
Natural versus human-caused coal fires
Some coal fires may start naturally through spontaneous combustion, lightning or wildfire.
Many modern coal-fire problems, however, are strongly associated with mining because extraction exposes
coal to air and creates extensive underground pathways.
Why coal fires migrate
A burning front consumes coal and transfers heat into adjacent material.
If nearby coal reaches ignition conditions, the combustion zone advances.
Collapse above burned-out areas may then open fresh cracks, allowing additional oxygen into the system.
This creates a destructive feedback:
burning → void formation → fracturing → oxygen supply → additional burning.
Underground Mine Fires
Mine fires overlap with coal seam fires but are not exactly the same phenomenon.
A mine provides an artificial underground architecture of:
- shafts;
- tunnels;
- fractured rock;
- ventilation routes;
- abandoned workings;
- exposed coal surfaces.
These openings can transport oxygen directly to combustible material.
In abandoned mines, incomplete maps and inaccessible tunnels make firefighting particularly difficult.
Why abandoned mines remain dangerous
Even when mining has stopped, underground workings can remain structurally unstable for decades.
If coal continues burning, the mine can become both a fire hazard and a ground-collapse hazard.
Peat Fires: The Zombie Fires Beneath Wetlands
Peat forms where partially decomposed plant material accumulates in waterlogged environments.
When peat dries sufficiently, it can ignite and smolder beneath the surface.
Why peat fires behave differently from wildfires
Flaming wildfires move rapidly across vegetation.
Peat fires usually propagate much more slowly through organic-rich soil.
But slow does not mean harmless.
Smoldering peat can:
- continue for weeks or months;
- burn deeply below the visible surface;
- produce enormous quantities of smoke;
- reignite vegetation above;
- consume the soil itself;
- lower the ground surface after organic material disappears.
Can peat fires survive winter?
In some circumstances, smoldering combustion can persist underground through cold periods and later
become active again at the surface.
Such overwintering fires are sometimes described as holdover fires or
zombie fires.
Landfill & Underground Waste Fires
Not every underground fire is geological in origin.
Landfills can contain enormous volumes of combustible material buried beneath layers of soil and waste.
Potential ignition mechanisms include:
- hot waste entering the landfill;
- chemical reactions;
- self-heating;
- surface fires propagating downward;
- equipment or industrial activity;
- gas-related combustion.
Why landfill fires are difficult
Adding oxygen while excavating can intensify combustion.
Flooding the site can create contaminated runoff.
Fire may also migrate through heterogeneous pockets of waste that are difficult to map.
These events belong at the intersection of subsurface fire, pollution and waste management.
For broader contamination issues, see
Pollution & Contamination Explained.
Natural Gas Fires & Eternal Flames
A flame emerging from the ground is not necessarily an underground coal fire.
In some locations, combustible gases migrate naturally toward the surface through fractures and porous
rock.
If ignited, those seeps may support persistent flames.
Natural eternal flames
Natural gas seeps have produced long-lived flames in several parts of the world.
Their fuel comes from continuously supplied hydrocarbons rather than a burning coal or peat body.
Gas craters
Gas-bearing ground can also produce spectacular burning craters when subsurface cavities collapse or
drilling and extraction disturb gas reservoirs.
These are related to underground fires by combustion, but their geology is fundamentally different from
a migrating coal-seam fire.
For more on gas escaping from Earth’s crust, see
Methane Seeps & Gas Hydrates.
Burning Mountains: When a Coal Fire Becomes a Landscape Feature
Some coal seams intersect hillsides and mountain slopes.
When such seams ignite, combustion can migrate slowly through the rock for extremely long periods.
The result may be:
- heated ground;
- mineral alteration;
- smoke vents;
- cracked slopes;
- burned or baked rock;
- vegetation anomalies.
These sites demonstrate that underground fire can function almost like an unusually slow geological
process, progressively altering the landscape as the combustion front advances.
How Do Underground Fires Spread?
Underground fire movement depends on the distribution of fuel and oxygen.
Along coal seams
Coal fires can migrate laterally or vertically where connected combustible material exists.
Through fractures
Cracks allow oxygen to penetrate deeper underground.
Through mine workings
Shafts and tunnels can act as ventilation systems, transporting air through areas that would otherwise
be oxygen-poor.
Through porous peat
Peat fires move through dry organic layers as slow smoldering fronts.
Through waste
In landfills, combustion can migrate through irregular zones containing plastics, paper, wood, organic
matter and other combustible materials.
Because underground fuel is rarely distributed uniformly, these fires frequently have complicated and
unpredictable boundaries.
Signs of an Underground Fire
Most underground combustion remains hidden.
Surface clues can include:
- persistent smoke or steam vents;
- unusually warm soil;
- burning or chemical odors;
- dead vegetation;
- discolored rock or soil;
- new fissures;
- localized subsidence;
- collapsing pavement;
- glowing cracks at night;
- snow melting unusually quickly over particular areas.
None of these signs alone proves that an underground fire is present.
Geothermal activity, leaking utilities, decomposition, industrial infrastructure and other processes can
produce some similar effects.
Proper diagnosis requires measurements.
Why Are Underground Fires Dangerous?
Fire itself is only one part of the problem.
Toxic gases
Incomplete combustion can produce dangerous concentrations of carbon monoxide and other gases.
Smoke and particulates
Persistent underground burning can degrade air quality across surrounding communities.
Extreme ground temperatures
Surface material above a fire may become dangerously hot even when flames are not visible.
Subsidence
Burning removes underground material and can destabilize the layers above it.
Sudden collapse
A surface that appears solid may overlie weakened ground or a burned-out cavity.
Infrastructure damage
Underground fires can affect:
- roads;
- railways;
- buildings;
- utility lines;
- mine infrastructure.
Underground Fires, Subsidence, Fissures & Ground Collapse
One of the most important geological consequences of underground combustion is
loss of support beneath the surface.
When coal or peat burns away, underground material is physically removed.
Surrounding layers may fracture, compact or collapse into the newly created void.
Surface effects may include:
- gradual sinking;
- irregular depressions;
- cracked roads;
- ground fissures;
- localized collapse holes;
- building damage.
Is this a sinkhole?
Not necessarily.
A classic geological sinkhole usually involves dissolution of soluble rock such as limestone, gypsum
or salt.
Collapse above a burned coal seam or mine is instead a combustion- or mining-related subsidence
failure.
For dissolution-driven collapse, see
Sinkholes & Dolines Explained.
For large-scale sinking, see
Land Subsidence Explained.
Smoke, Toxic Gases & Pollution
Underground combustion can transform an apparently local geological curiosity into a serious environmental
problem.
Emissions may include varying amounts of:
- carbon dioxide;
- carbon monoxide;
- particulate matter;
- sulfur compounds;
- nitrogen compounds;
- volatile organic compounds;
- trace metals and other contaminants depending on the fuel.
Why ventilation is dangerous
Cracks that allow smoke to escape also allow oxygen to enter.
This means the visible vent can be part of the mechanism sustaining the fire.
Groundwater contamination
Heat and altered minerals can affect water chemistry around some underground fires, while firefighting
operations themselves may mobilize contaminants.
How Scientists Detect Underground Fires
Mapping an underground fire is difficult because its most important features are hidden.
Investigators therefore combine several methods.
Temperature measurements
Elevated ground temperatures can reveal combustion zones near the surface.
Gas measurements
Monitoring gases escaping through vents or boreholes can help locate active burning.
Boreholes
Drilling allows direct measurements of subsurface temperature and gas concentrations.
Geophysical surveys
Changes in subsurface properties may help map burned zones, cavities and altered material.
Repeated ground surveys
Measuring deformation can reveal areas where burning has caused subsidence.
Satellite & Thermal Monitoring of Underground Fires
Large underground fire zones can sometimes produce detectable thermal anomalies at Earth’s surface.
Satellite thermal imagery allows scientists to identify hot areas and track changes over time.
What satellites can reveal
- persistent surface heat;
- expansion of thermal anomalies;
- new hot spots;
- seasonal changes;
- large coal-fire regions that are difficult to survey from the ground.
What satellites cannot tell us alone
A thermal anomaly does not automatically identify its cause.
Wildfires, industrial activity, geothermal heat and other processes may produce elevated temperatures.
Satellite data therefore work best when combined with geological mapping and field measurements.
Can Underground Fires Be Extinguished?
Sometimes.
But large underground fires can be among the most difficult combustion hazards to control.
Excavation
Burning material may be physically removed when the fire is sufficiently shallow and accessible.
Isolation trenches
Trenches can sometimes separate burning material from unburned fuel.
Flooding or water injection
Water may cool burning zones, although delivering enough water to the correct underground location can be
difficult.
Inert gas or sealing
Reducing oxygen availability can suppress combustion in some mines or enclosed zones.
Grouting and barriers
Engineered barriers may be used to restrict airflow or isolate parts of an underground fire.
Why some fires are left burning
For enormous coal-field fires, complete extinguishment may require excavating or isolating such large
volumes of material that it becomes technically or economically impractical.
Management may instead focus on containing the fire, monitoring its movement and keeping people away from
hazardous ground.
Famous Underground Fires Around the World
Underground fires occur on multiple continents, but several sites have become especially important because
they illustrate different mechanisms.
| Location | Fire Type | Why It Is Famous |
|---|---|---|
| Centralia, Pennsylvania | Mine / coal seam fire | Long-running fire associated with the abandonment of much of a town |
| Jharia, India | Coalfield fires | Extensive network of burning coal seams and mining-related ground instability |
| Burning Mountain, Australia | Natural coal-seam fire | Extremely long-lived migrating underground combustion |
| Darvaza, Turkmenistan | Natural-gas fire | Persistent burning gas crater commonly known as the Door to Hell |
Centralia, Pennsylvania: The Mine Fire Beneath a Town
Centralia became the world’s best-known modern example of an underground coal fire
affecting an inhabited community.
A fire that began in the early 1960s spread into abandoned coal-mine workings beneath the Pennsylvania town.
Over time, concerns grew over:
- toxic gases;
- surface heating;
- ground fissures;
- subsidence;
- potential collapse.
Most residents eventually relocated, leaving Centralia synonymous with the idea of a town sitting above
an underground fire.
Why Centralia is geologically important
Centralia shows how mining infrastructure can allow a relatively localized ignition to spread into a
much larger subsurface system.
It also illustrates why underground-fire hazards can continue long after their initiating event has been
forgotten.
Jharia Coalfield, India: Fires Beneath One of the World’s Major Coal Regions
The Jharia coalfield is not a single underground fire.
It is a large coal-mining region affected by numerous persistent coal fires.
In some areas, burning seams have produced:
- smoke vents;
- hot ground;
- air pollution;
- subsidence;
- dangerous fissures;
- damage to homes and infrastructure.
Jharia illustrates the scale underground fire problems can reach when extensive coal deposits,
long-term mining and subsurface airflow interact.
Burning Mountain, Australia: A Natural Coal Fire Beneath a Mountain
Burning Mountain in New South Wales is one of the most remarkable examples of naturally occurring
underground coal combustion.
A burning coal seam moves slowly beneath the surface, leaving altered ground behind it.
Why Burning Mountain matters
Unlike many famous coal fires linked to modern mining, Burning Mountain demonstrates that underground coal
combustion can also occur as a natural geological phenomenon.
Over long periods, heat generated below the surface can alter rock, soil and vegetation while the active
combustion zone gradually migrates.
Darvaza Gas Crater: The Door to Hell
The glowing Darvaza gas crater in Turkmenistan is often grouped with the world’s famous
underground fires.
It is better understood as a long-lived natural-gas combustion site rather than a
coal-seam or peat fire.
The crater sits in a hydrocarbon-rich region and has burned for decades.
Its precise early history has been reported in different ways, so simplistic versions of the familiar
drilling-accident story should be treated cautiously.
Why it belongs here
Darvaza demonstrates the same basic principle seen in other persistent underground fires:
as long as combustible material and oxygen continue reaching the combustion zone, fire can persist
far longer than people expect.
Environmental Consequences of Underground Fires
Underground fires can reshape landscapes as well as pollute them.
Loss of soil and organic material
Peat fires can physically consume portions of the soil profile.
Rock alteration
Prolonged heating can bake, fracture and chemically alter surrounding rock.
Vegetation loss
Heat, toxic gases and soil changes can kill plants above active fires.
Ground deformation
Burning and collapse can create depressions and fracture networks.
Habitat degradation
Persistent smoke, altered drainage and unstable terrain can change ecosystems long after visible flames
have disappeared.
Underground Fires & Climate
Underground fires release carbon that had been stored in coal, peat or other organic material.
Peat fires are particularly important because peatlands can contain large stores of carbon accumulated
over long periods.
When that material burns, carbon is transferred into the atmosphere while the peatland itself may lose
part of its capacity to store carbon in the future.
Drying and fire risk
Peat is most resistant to burning when saturated.
Drainage, drought and prolonged dry conditions can expose deeper organic material to oxygen and increase
its susceptibility to fire.
Coal fires and emissions
Coal-seam fires also represent uncontrolled fossil-fuel combustion and can release greenhouse gases and
air pollutants without providing useful energy.
Common Myths About Underground Fires
“Smoke coming from the ground means volcanic activity.”
Usually not. Coal, peat, waste and leaking gases can all produce smoke or steam without magma being present.
“Rain will extinguish an underground fire.”
Surface rain may have limited effect when combustion occurs deep below ground or inside insulated material.
“If there are no flames, the fire is gone.”
False. Smoldering combustion can continue underground with little or no visible flame.
“All burning mountains are volcanoes.”
No. Some burning mountains contain coal seams or gas seeps rather than magma.
“Underground fires cause tectonic earthquakes.”
Underground combustion can produce cracking, subsidence and localized collapse, but that is fundamentally
different from tectonic earthquake generation.
“Every hole near an underground fire is a sinkhole.”
No. Collapse above mines or burned-out seams has a different mechanism from karst sinkholes produced by
dissolution of soluble bedrock.
Safety Around Burning Ground
Ground affected by an underground fire can be dangerous even when nothing dramatic is visible.
- Do not walk across smoking or cracked ground.
- Do not enter fissures, vents or abandoned mine openings.
- Do not assume cool-looking ground is structurally sound.
- Keep children and animals away from suspected fire zones.
- Avoid breathing smoke or gases emerging from vents.
- Report newly discovered burning ground to appropriate local authorities.
Carbon monoxide is particularly dangerous because it is colorless and odorless.
A dramatic plume of smoke may attract photographers, but a hidden cavity or toxic gas concentration is
far more important than the photograph.
Quick Classification: What Kind of Burning Ground Is It?
| What you observe | Possible explanation |
|---|---|
| Smoke vents in a coal-mining region | Coal seam or mine fire |
| Smoldering organic soil after wildfire | Peat fire |
| Heat and smoke inside a landfill | Subsurface waste fire |
| Persistent flame from a natural rock fracture | Combustible gas seep |
| Hot cracked ground above abandoned mine workings | Mine fire with subsidence |
| Steam and sulfur gases in a volcanic field | Potential geothermal or volcanic activity rather than combustion |
Correct classification matters because visually similar phenomena can have completely different causes.
Frequently Asked Questions About Underground Fires
What causes underground fires?
Underground fires can begin when coal, peat, buried waste or other combustible material is ignited.
Causes include spontaneous combustion, mining activity, wildfire, lightning, burning waste and other
human ignition sources.
How long can an underground fire burn?
Underground fires can burn for months, decades or considerably longer when large fuel deposits and a
continuing oxygen supply are available.
Can underground fires burn for hundreds of years?
Yes. Some natural coal-seam fires are thought to have persisted for very long periods because combustion
migrates slowly through extensive underground coal deposits.
Why are underground fires so hard to extinguish?
The burning zone may be deep, inaccessible and irregular, while fractures and mine tunnels continuously
supply oxygen. Fires can also migrate into areas that have not yet been treated.
What is a coal seam fire?
A coal seam fire is combustion occurring within a natural coal deposit. It may start naturally or because
of mining and other human activity and can migrate slowly through connected coal underground.
What is the difference between a coal seam fire and a mine fire?
A coal seam fire burns within a coal deposit. A mine fire occurs within active or abandoned mine workings.
In coal mines the two processes can overlap because a mine fire may ignite surrounding coal seams.
Can peat burn underground?
Yes. Dry peat can smolder below the surface for long periods. Peat fires may remain active even after
visible surface flames disappear.
Can underground fires survive winter?
Some smoldering peat and organic-soil fires can persist beneath insulating soil or snow and become active
again when conditions change.
Are underground fires volcanic?
Usually not. Most underground fires involve coal, peat, buried waste or combustible gas. Volcanic and
geothermal systems can also produce hot ground and gas vents, but the heat source is different.
Can underground fires cause sinkholes?
Underground fires can cause subsidence and localized collapse by removing or weakening material below
the surface. These collapse features should not automatically be classified as geological sinkholes,
which commonly form through dissolution of soluble rock.
Can underground fires cause earthquakes?
Underground fires can cause localized cracking, subsidence and collapse but do not generate major
tectonic earthquakes.
How are underground fires detected?
Detection methods include ground-temperature measurements, gas monitoring, boreholes, thermal cameras,
geophysical surveys, drones and satellite thermal imagery.
What is the most famous underground fire?
Centralia in Pennsylvania is probably the best-known modern underground mine-fire example. The Jharia
coalfield in India is one of the largest and most important coal-fire regions, while Burning Mountain in
Australia is a famous natural coal-seam fire.
Why does smoke come out of cracks above underground fires?
Fractures connect the burning zone with the atmosphere. Hot combustion gases rise through these pathways,
while some cracks simultaneously allow fresh oxygen to enter the fire.
Is the Darvaza Door to Hell an underground fire?
Darvaza is a persistent natural-gas fire in a crater rather than a classic coal-seam, mine or peat fire.
It is nevertheless closely related to the broader phenomenon of long-lived subsurface-fueled combustion.
